It will be apparent to the reader that there is much to learn about the pathogenesis of YF. The role of specific genes and molecular determinants of neurotropism and viscerotropism has been defined only partially. The availability of infectious clones and a small animal (hamster) model should allow dissection of virulence factors, which can then be tested in the more difficult monkey model. The marked differences between wild-type YF strains should be evaluated by evaluating the relationships between virulence and genome sequence. The role of cytokine dysregulation and endothelial injury in YF will be elucidated as access to patients and of patients to more sophisticated medical care improves. The number of cases of YF in unvaccinated travelers hospitalized after return from the tropics has unfortunately increased, but such cases afford unique opportunities to study the pathogenesis of renal failure, coagulopathy, vascular instability, and shock, as well as new treatment modalities. At the cellular level, there are also important opportunities for research on YF virus-cell receptor interactions, the control of apoptotic cell death, and the predilection for cells of the midzone of the liver lobule. The role of dendritic cells in the early stage of YF infection is deserving of study. Finally, the role of the immune response to infection, particularly cellular immunity, is poorly characterized, and the suggestion that immune clearance may aggravate the condition of the host during the period of intoxication should be evaluated in appropriate animal models.